Pt-Pd diesel oxidation catalyst with CO/HC light-off and HC storage function

Abstract

The present invention is directed to a diesel oxidation catalyst for the treatment of exhaust gas emissions, such as the oxidation of unburned hydrocarbons (HC), and carbon monoxide (CO) and the reduction of nitrogen oxides (NOx). More particularly, the present invention is directed to a novel washcoat composition comprising two distinct washcoat layers containing two distinctly different ratios of Pt:Pd.

1. An emission treatment system for treating exhaust gas emissions from a diesel engine containing hydrocarbons, carbon monoxide, and particulate matter comprising:
 a diesel oxidation catalyst comprising a carrier substrate comprising a bottom washcoat layer coated on said carrier substrate comprising a high-surface area support material and a precious metal catalyst consisting of platinum (Pt) and palladium (Pd) in a Pt:Pd weight ratio, wherein said support is a substantially silica-free high-surface area support material, and wherein said bottom washcoat layer does not contain a hydrocarbon storage component; a top washcoat layer coated over said bottom washcoat layer comprising a high-surface area support material, one or more hydrocarbon storage components, and a precious metal catalyst consisting of platinum (Pt) and palladium (Pd) in a Pt:Pd weight ratio; and wherein said Pt:Pd weight ratio in said top washcoat layer is from about 2:1 to up to about 4:1 and is greater than said Pt:Pd weight ratio of said bottom washcoat layer; 
 a soot filter located downstream from the diesel oxidation catalyst to trap particulate matter from the exhaust gas; and 
 a catalyst for the conversion of NOx located downstream from the soot filter. 
 
     
     
       2. The system of  claim 1 , wherein said Pt:Pd ratio of said bottom washcoat layer of the diesel oxidation catalyst is from less than about 2:1 to about 1:2. 
     
     
       3. The system of  claim 1 , wherein said Pt:Pd ratio of said bottom washcoat layer of the diesel oxidation catalyst is 7:5 and said Pt:Pd ratio of said top washcoat layer is 3:1. 
     
     
       4. The system of  claim 1 , wherein said bottom washcoat and/or top washcoat layer of the diesel oxidation catalyst further comprises zirconium oxide. 
     
     
       5. The system of  claim 1 , wherein said bottom washcoat layer of the diesel oxidation catalyst further comprises lanthanum oxide. 
     
     
       6. The system of  claim 1 , wherein said support material of said bottom washcoat layer of the diesel oxidation catalyst is selected from the group consisting of alumina, zirconia, titania and mixtures thereof. 
     
     
       7. The system of  claim 1 , wherein said support material of said top washcoat layer of the diesel oxidation catalyst is selected from the group consisting of silica, alumina, zirconia, titania and mixtures thereof. 
     
     
       8. The system of  claim 1 , wherein said hydrocarbon storage component of the diesel oxidation catalyst is a zeolite and said zeolite is selected from the group consisting of faujasite, chabazite, clinoptilolite, mordenite, silicalite, zeolite X, zeolite Y, ultrastable zeolite Y, ZSM zeolite, offretite, and beta zeolite. 
     
     
       9. An emission treatment system for treating exhaust gas emissions from a diesel engine containing hydrocarbons, carbon monoxide, and particulate matter comprising:
 a diesel oxidation catalyst comprising a carrier substrate; an upstream washcoat layer coated over an upstream portion of said carrier substrate comprising a high-surface area support material, one or hydrocarbon storage components, and a precious metal catalyst consisting of platinum (Pt) and palladium (Pd) in a Pt:Pd weight ratio; a downstream washcoat layer zone coated over a downstream portion of said carrier substrate comprising a high-surface area support material and a precious metal catalyst consisting of platinum (Pt) and palladium (Pd) in a Pt:Pd weight ratio, wherein said support is a substantially silica-free high-surface area support material, and wherein said downstream washcoat layer does not contain a hydrocarbon storage component; and wherein said Pt:Pd weight ratio in said upstream washcoat layer is from about 2:1 to up to about 4:1 and is greater than said Pt:Pd weight ratio of said downstream washcoat layer; 
 a soot filter located downstream from the diesel oxidation catalyst to trap particulate matter from the exhaust gas; and 
 a catalyst for the conversion of NOx located downstream from the soot filter. 
 
     
     
       10. The system of  claim 9 , wherein said Pt:Pd ratio of said downstream washcoat layer of the diesel oxidation catalyst is from less than about 2:1 to about 1:2. 
     
     
       11. The system of  claim 9 , wherein said Pt:Pd ratio of said upstream washcoat layer of the diesel oxidation catalyst is 3:1 and said Pt:Pd ratio of said downstream washcoat layer is 7:5. 
     
     
       12. The system of  claim 9 , wherein said hydrocarbon storage component of the diesel oxidation catalyst is a zeolite and said zeolite is selected from the group consisting of faujasite, chabazite, clinoptilolite, mordenite, silicalite, zeolite X, zeolite Y, ultrastable zeolite Y, ZSM zeolite, offretite, and beta zeolite. 
     
     
       13. The system of  claim 9 , wherein said substrate of the diesel oxidation catalyst has an upstream edge and a downstream edge and said top upstream washcoat layer is zone coated over a length from about 5% to about 70% of said substrate from said upstream edge. 
     
     
       14. The system of  claim 12 , wherein said bottom downstream washcoat layer of the diesel oxidation catalyst is zone coated over a length from about 95% to about 30% of said substrate from said downstream edge. 
     
     
       15. A method for treating a diesel exhaust gas stream containing hydrocarbons, carbon monoxide and particulate matter, the method comprising:
 (a) providing a diesel oxidation catalyst comprising: 
 (i) a carrier substrate having an upstream edge and a downstream edge; 
 (ii) a bottom washcoat layer coated on said carrier substrate comprising a high-surface area support material and a precious metal catalyst consisting of platinum (Pt) and palladium (Pd) in a Pt:Pd weight ratio, wherein said support is a substantially silica-free high-surface area support material, and wherein said bottom washcoat layer does not contain a hydrocarbon storage component; 
 (iii) a top washcoat layer coated over said bottom washcoat layer comprising a high-surface area support material, one or hydrocarbon storage components, and a precious metal catalyst consisting of platinum (Pt) and palladium (Pd) in a Pt:Pd weight ratio; and 
 (iv) wherein said Pt:Pd weight ratio in said top washcoat layer is from about 2:1 to up to about 4:1 and is greater than said Pt:Pd weight ratio of said bottom washcoat layer; 
 (b) contacting said diesel exhaust gas stream with said diesel oxidation catalyst from said upstream edge to said downstream edge for the treatment of exhaust gas emissions so that the oxidation catalyst oxidizes hydrocarbons and carbon monoxide in the exhaust gas to carbon dioxide and water; and 
 (c) conveying the exhaust gas exiting the diesel oxidation catalyst to a downstream soot filter and NOx conversion catalyst. 
 
     
     
       16. The method of  claim 15 , wherein said Pt:Pd ratio of said bottom washcoat layer of the diesel oxidation catalyst is 7:5 and said Pt:Pd ratio of said top washcoat layer is 3:1. 
     
     
       17. The method of  claim 15 , wherein the soot filter comprises a catalyzed soot filter (CSF). 
     
     
       18. The method of  claim 17 , wherein the NOx conversion catalyst comprises a selective catalytic reduction (SCR) component located downstream of the catalyzed soot filter (CSF).